HOW ACCURATE ARE THE FACTORY TACHS

I was looking at the replacement tach at Classic industries that has modern internals. I understand they are more accurate than the original and look the same. I don't have a factory tach option.
I put one of the Classic Industries reproduction tach in my 68 Barracuda. I had seen some online comments that they were jumpy and not accurate. However mine works fine. The response is smooth and the rpm matches to my shift light. The part has a one year warranty so if it is bad out of the box or in use you have some coverage.